The carpal tunnel is a semi-rigid space bound by bone and ligament which runs through the wrist. The median nerve and nine flexor tendons which control movement of the thumb and fingers of the hand go through this tunnel. The abductor pollicis brevis is an intrinsic muscle of the wrist-thumb region of the hand that is exclusively controlled by the median nerve. Injuries and conditions such as Carpal Tunnel Syndrome ("CTS"), some types of neck and elbow injuries as well as other hand disorders affect the innervation of the abductus pollicis brevis. Assessment of this muscle's strength is, thus, an important diagnostic tool in testing for some types of neck and elbow injuries, and for CTS and other hand disorders.
Currently, the strength of the abductor pollicis brevis is subjectively graded by manual testing procedures. Specifically, in conventional diagnostic testing, physicians rely on the patient's history and subjective tests such as the Tinel's Sign, the Phalen's Test, and general sensation. An additional subjective test for measuring thenar abduction strength has been utilized, wherein the physician feels the abduction resistance of the patient's thumb and applies a qualitative rating. A comparison of the affected and unaffected sides provides a basis for the physician's qualitative rating. As with the above-referenced tests, this test does not quantitatively measure anything that can be readily reproduced.
An objective diagnostic test of the median nerve has been developed which measures the conduction status of the median nerve. Slowed conduction along the median nerve is usually interpreted as a sign of nerve damage which may be indicative any of the above-referenced injuries. However, as with other diagnostic tests, nerve conduction studies may have false-positive and false-negative results.
In an article by Robin R. Richards, et al. entitled Measurement of Wrist, Metacarpophalangeal Joint, and Thumb Extension Strength in a Normal Population, an objective test is disclosed where the hand positioned with the wrist in a neutral flexion-extension position and the ulnar aspect of the hand firmly positioned on a flat surface of a jig. A measuring device such as a myometer is placed perpendicular to the thumb just distal to the interphlangal joint of the thumb. The patient extends the thumb with maximal force against the myometer head. In this approach the hand is manually restrained to prevent changes in position that might aid in thumb extension. The test disclosed in the Richards, et al. article provides for a quantitative measurement of wrist and thumb extension. The measurement obtained from this test includes muscles not exclusively innervated by the median nerve.
Quantitative measurements of the abductor pollicis brevis strength is necessary for evaluation of procedures effecting the median nerve. Existing devices do not allow rapid, painless, noninvasive and quantitative measurements of thenar abduction resistive strength.
It is important for physicians to be able to quantitatively measure any deterioration of median nerve function over time. The ability to quantitatively measure abduction strength of the abductor pollicis brevis allows the physician to objectively determine whether or not deterioration of the median nerve has occurred. Such a determination assists the physician in evaluating whether or not preliminary treatment of an injury affecting the median nerve is effective or whether an alternative treatment such as an invasive surgical procedure is required. In addition, to perform meaningful treatment outcome studies it is important to be able to compare the pretreatment versus the post treatment strength of the median innervated abductor pollicis brevis muscle. For these studies, qualitative measurement of abduction strength is essential.